The ultimate goal of this project is to develop a Positron Emission Tomography (PET) radiotracer for brain imaging of the cyclic nucleotide phosphodiesterase 10A (PDE10A). PDE10A is specifically expressed in the brain with high levels in striatal medium-sized spiny projection neurons (MSN) where it plays a critical role in the regulation of both cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP). Abnormal striatal levels of PDE10A affect striatal output and may contribute substantially to the pathophysiology of schizophrenia, Huntington's disease (HD) and other mental disorders. Decreased striatal PDE10A level has been correlated with the severity of HD, while inhibition of PDE10A has been proposed as a novel therapeutic strategy for treating schizophrenia and related conditions. Thus, a PET radiotracer for PDE10A would be a valuable tool for clinical neuroscience research. To achieve the goal of this project, we first radiolabeled a representative PDE10A compound, 2-((4-(1-[11C]methyl-4-(pyridin-4-yl)-1H-pyrazol-3- yl)phenoxy)-methyl)quinoline ([11C]MP-10) with C-11. MP-10 is a high potency PDE10A inhibitor (IC50 = 0.37 nM) with high selectivity (1000 fold) for PDE10A vs. other PDEs and CNS receptors. We also successfully modified the structure of MP-10 to make a fluorine analogue, TuJF103 (IC50 = 0.27 nM). Preliminary biodistribution evaluation of [11C]MP-10 and [18F]TUJF103 in rats, and microPET imaging studies of both radiotracers in monkeys displayed good contrast between the target (striatal) and the reference (cerebellum) region. However, analysis of the time-activity curves of striatum and cerebellum and subsequent metabolite analysis of rat brain and blood indicated the presence of lipophilic radiolabeled metabolites that accumulate non-specifically in the brain. Such metabolites could limit the clinical utility of either [11C]MP-10 or [18F]TUJF103 as novel PET tracers for imaging PDE10A. To overcome such concerns, this proposal will optimize the structure of MP-10 to synthesize new analogues with high affinity and selectivity for PDE10A; radiolabel lead candidates with C-11 or F-18 and then use in vivo methods to validate optimal PET radiotracers for imaging PDE10A in the brain. Consequently, the specific aims of the R21 component include: (1) synthesize new analogues by structural optimization of MP-10; (2) measure the affinities of new analogues in vitro; (3) radiolabel the ligands having high affinities (IC50<15 nM) and high selectivity (> 100 fold) with C-11 or F-18; (4) conduct biodistribution and brain uptake studies of radiotracers in rats to identify at least two promising candidates. The specific aims of the R33 component will be the continued evaluation of these candidate radiotracers in primates with the goal of identifying a PET tracer suitable for translational clinical evaluation for imaging PDE10A in the brain with PET.